/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function levelOrder(root: TreeNode | null): number[] {
    const res: number[] = [];

    let h = 0, t = 0;
    let queue: number[] = new Array<number>();
    const d = new Array<TreeNode>();
    if (root) {
        d[t++] = root;
    }

    while (h < t) {
        const tt = d[h++];
        res.push(tt.val)
        if (tt.left != null) d[t++] = tt.left
        if (tt.right != null) d[t++] = tt.right

    }

    return res;
};